The present invention relates in general to a sound detecting circuit of a television receiver and in particular to an IF filter.
Ordinarily, in a television receiver, the signals from an antenna are transformed to an intermediate frequency (IF) by a tuner. For example, a picture carrier included in the intermediate frequency signal is transformed to 58.75 MHz and a sound carrier to 54.25 MHz. These transformed picture and sound carriers are supplied to an IF amplifier and after amplification to a predetermined level, the IF amplifier generates a second sound IF signal at a beat frequency, for example, 4.5 MHz between the IF picture carrier and the IF sound carrier. Namely, the sound carrier is frequency-transformed with the picture carrier acting as a local oscillator. This transformation is called the first sound detection. The second sound IF signal of 4.5 MHz is then FM-demodulated (in a second sound detection) to regenerate the original sound signal. As shown in FIG. 1(a), the IF signal is supplied to an IF amplifier 2 through an input terminal 1 and is amplified to a prescribed level. This amplified IF signal is then applied to a video detector 3 which simultaneously performs the video detection and the sound detection. Next, the 4.5 MHz signal is eliminated from the output of the video detector 3 by a trap circuit 4 tuned to the beat frequency of 4.5 MHz and the signal is output on a video output terminal 5. Since the output of the video detector 3 also includes the second sound IF signal of 4.5 MHz at the same time, this signal is extracted with a 4.5 MHz filter 6 which is output on a sound IF output terminal 7. The resultant circuit as shown in FIG. 1(a) is simple, but it suffers from disadvantages. The IF signal includes not only the picture carrier and the sound carrier but also a color subcarrier. This color subcarrier is also amplified and because of the nonlinearity of the video detector, an intermodulation is effected between these carriers. As a result, a so-called 920 kHz beat is generated. Consequently, in the case where more importance is attached to video quality than to sound quality, the sound carrier level in the IF signal at the input terminal 1 is lowered in order to decrease the 920 kHz beat. However, as a result the sound signal-to-noise ratio (S/N) disadvantageously decreases. For this reason, the circuit of FIG. 1(b) is used to overcome the defect of the circuit of FIG. 1(a). Between the IF amplifier 2 and the video detector 3, which is the main source of the 920 kHz beat, a sound carrier trap circuit 8 is provided to attenuate the 920 kHz beat of the video signal output. Also, the beat frequency at 4.5 MHz which is the difference between the picture carrier and sound carrier is obtained by inputting the IF amplifier output into a first sound detector 9.
Recently, sound multiplexing has been implemented and a strong demand has arisen not only for a high quality picture but also for high quality sound. Noise, the so-called buzz sound, and the sound S/N determined by the synchronous sound and video signals have consequently become important issues.
The conventional methods of FIGS. 1(a) and (b) do not produce satisfactory sound S/N and need to be improved. If a separate carrier method is used in which the sound signal system is separated from the video signal system and separate processing is performed, the problem of sound S/N is improved but this method had the disadvantage that the circuits become complicated and cost much more.